• 한국분말재료학회지
• /
• 제28권4호
• /
• pp.317-324
• /
• 2021

High-strength low-alloy (HSLA) steels show excellent toughness when trace amounts of transition elements are added. In steels, prior austenite grain size (PAGS), which is often determined by the number of added elements, is a critical factor in determining the mechanical properties of the material. In this study, we used two etching methods to measure and compare the PAGS of specimens with bainitic HSLA steels having different Nb contents These two methods were nital etching and picric acid etching. Both methods confirmed that the sample with high Nb content exhibited smaller PAGS than its low Nb counterpart because of Nb's ability to hinder austenite recrystallization at high temperatures. Although both etching approaches are beneficial to PAGS estimation, the picric acid etching method has the advantage of enabling observation of the interface containing Nb precipitate. By contrast, the nital etching method has the advantage of a very short etching time (5 s) in determining the PAGS, with the picric acid etching method being considerably longer (5 h).

• 한국재료학회지
• /
• 제13권10호
• /
• pp.683-690
• /
• 2003

The high strength low alloy(HSLA) steels microalloyed with Nb, Ti and V have been widely used as the automobile parts to decrease weight of vehicles. The effects of process conditions are investigated in the aspects of the precipitation behavior and the mechanical properties of HSLA steel microalloyed with Nb and Ti using TEM, SANS and mechanical testing. When Ti was added to a 0.07C-1.7Mn steel which was coiled at $500^{\circ}C$, the specimen revealed the property of higher tensile strength of 853.1 MPa and the stretch-flangeability of 60%. The stretch-flangeability was increased up to 97.8% for coiling temperature above $700^{\circ}C$. The precipitation hardening cannot be achieved in the 0.045C-1.65Mn steel which was the lower density of fine precipitates. However, the 0.07C-1.7Mn steels containing Nb and/or Ti which was coiled at X$/^{\circ}C$ have a high precipitates density of $2${\times}$10^{ 5}$/$\mu$㎥. The high strength of these steels was attributed to the precipitation hardening caused by a large volume froction of (Ti, Nb)C precipitates with a size below 5 nm in ferrite matrix.

• 열처리공학회지
• /
• 제37권4호
• /
• pp.155-162
• /
• 2024

SCr420H steel which is commonly utilized for automotive components requires the carburizing heat treatment process. Abnormal grain growth during this treatment significantly affects the mechanical properties of the steel parts. Consequently, a process designed to prevent abnormal grain growth at certain elevated temperatures is essential. For enhanced grain refinement, we considered the addition of Nb in SCr420H steel. The experimental condition of the carburizing heat treatment involved reheating the steel sample to temperatures between 940℃ and 1080℃. Using scanning electron microscopy, we examined the microstructure of specimens treated with the secondary solution, revealing an organization of bainite and ferrite. Transmission electron microscopy was utilized to determine the type, shape, and size of the carbonitrides, showing a high fraction of AlN at the secondary solution treatment temperature of approximately 1050℃ and of (Nb,Ti)(C,N) around 1200℃. AlN particles measured about 100 nm and (Nb,Ti)(C,N) about 50 nm. Optical microscopy was utilized to assess grain size variations at different secondary solution treatment temperatures. It is noted that the temperature at which abnormal grain coarsening occurred rose with increasing secondary solution treatment temperatures, indicating a greater influence of (Nb,Ti)(C,N) with higher heat treatment temperatures. This research provides reference data for preventing abnormal grain growth in Nb-added low alloy steels undergoing carburizing heat treatment.